Rhodium plating



Patented Dec. 4, 1951 RHODIUM PLATING Frank Herbert Reid, Hounslow, England, assi nor to The International Nickel Company Inc., New York, N. Y., a corporation of Delaware No Drawing. Application August 18, 1948, Serial No. 44,979. In Great Britain August 19, 1947 Claims. (Cl. 20447) This invention relates to the electro-deposition of rhodium onto other metals and to solutions for use therein.

The electrodeposition of rhodium onto other metals for decorative or tarnish-resisting purposes is commonly effected with the use of electrolytes of the kind containin as the essential constituent rhodium sulphate or rhodium phosphate or a mixture of those two compounds together with varying proportions of free sulphuric acid or free phosphoric acid or a mixture thereof. The free acid is necessary to prevent hydrolysis of the rhodium compounds in the electrolyte with consequent precipitation of rhodium hydroxide.

I have found that electrolytes of this kind, when operated under normal plating conditions, tend to furnish rhodium deposits which become milky in appearance as the thickness increases. The exact thickness of the deposit at which the desired metallic brilliance is lost varies with the composition of the electrolyte and the operating conditions. Thus, deposits from phosphate electrolytes may become milky when they are only 2.5 millionths of an inch thick whereas deposits from phosphate-sulphate electrolytes may remain bright until they are as thick as millionths of an inch. Deposits from sulphate electrolytes generally remain bright until they are about 5 to 7 millionths of an inch thick. Now the minimum thickness generally regarded as satisfactory in industrial practice is 5 millionths of an inch and it is desirable that the deposits should be two or three times as thick as this.

An object of this invention is to provide a rhodium-plating process by which light deposits thicker than the prior art deposits can be produced.

Another object of this invention is to produce improved rhodium deposits by means of a critical amount of lead in the electrolyte.

A further object of the invention is to provide a novel lead-containing electrolyte for use in rhodium plating.

I have discovered that if a critical amount of lead is added to the electrolyte in the form of any convenient acid-soluble compound of lead, e. g. the nitrate of acetate, the thickness of the rhodium deposits is greatly increased before their characteristic brilliance is lost or they become milky in appearance.

The amount of lead lies between 0.001 and 0.02 gram per liter. Below a concentration of 0.001 gram per liter the lead is practically inefiective in increasing the thickness before milkiness appears and if the lead concentration exceeds 0,02

gram per liter, the deposits tend to show milky streaks at thicknesses little, if any, greater than when no lead is used.

It is to be understood that bright deposits greater than 5 millionths of an inch thick can be obtained with some electrolytes under suitable operating conditions, but even then the use of lead is advantageous because the deposits should as a general rule be thicker than the minimum and the risk of milkiness appearing before the desired thickness is reached can be eliminated. It is also to be understood that even with the use of lead the deposits cease to be bright at thicknesses which vary with the composition of the electrolyte and the operating conditions.

In processes of the kind in question as carried on hitherto, it is usual to employ a rhodium concentration of 2 grams per liter of electrolyte, a current density of from 2 to 4 amperes per square decimeter and an operating temperature of from room temperature to about C. but more usually from 35 to 45 C. In carrying out the present invention none of these figures is critical, but it is preferred to depart from the prior art practice only to the extent of adding the critical amount of lead. Nevertheless, if desired the rhodium concentration may vary between, say, about 0.5 and about 5 grams per liter. The free acid must always be enough to prevent hydrolysis of rhodium compounds, as those skilled in the art will understand. Increase in the free acid content above, say, 20 milliliters of sulphuric acid per liter decreases the current efliciency and so is time-consuming and uneconomic. The current density must, of course, be high enough to ensure that the metal on which the deposit is being made will not be chemically attacked. but may be less than 2 amps./dm. High cathodic current densities, say above about 5 amps/dmfi, tend to cause deterioration of the electrolyte, so I prefer to avoid their use.

I have found that if the operating temperature exceeds 40 C. the deposit becomes milky at progressively smaller thicknesses as the lead concentration approaches 0.02 gram per liter. To obtain the best results, therefore. the higher temperatures and concentrations should not be used together.

EXAMPLE I An electrolyte of the phosphate-sulphate type was prepared by dissolving rhodium hydroxide in phosphoric acid and adding the phosphate solution to dilute sulphuric acid, the resulting solution containing 2 grams of rhodium per liter andZO milliliters of concentrated sulphuric acid per liter. On working with a current density of 4 amps./dm. the results shown in Table I were obtained.

Table I Thick- NO 63? Temp Time ness of Appearance 0.) (mins.) Rh (ins. of deposit (gum/1) X 10- n11 20 7 14 ilky. 1111 60 3 10 Blushed O. 001 20 30 35 Bright. 0. 001 60 10 42 D0. 0. 005 20 20 21 D0. 0. 005 60 10 35 D0. 0. 05 20 2O Severely:

streaked EXAMPLE II An electrolyte of the sulphate type was pre-' pared by dissolving finely divided rhodium in concentrated sulphuric acid, and diluting thesolution to give aconcentra tion of'2 grams of rhodium per liter and 10 milliliters of concentrated sulphuric acid per liter. On working with'acurrent density of 4 arnps./dm. the results shown in Table II were obtained:

An electrolyte of the phosphate type was prepared by dissolving rhodium hydroxide in phosphoric acid and diluting the solution to give a concentration of 2 grams of rhodium per liter and milliliters of phosphoric acid per liter. This solution Was electrolysed with a current density of 4 amps./dm. and gave the results shown in- Table III.

Table III Lead Thick- No comm Temp. Time ncssof Appearance C.) (mms) R11 (1118 of deposit 15 nil 20 2 2.5 S lig lg tly mi l6 0. 005 20 15 Bright? The metals onto which rhodium may be plated according to the invention are any that are compatible with the acid electrolytes and include, besides nickel, nickel silver, gold, platinum, palladium, copper and brass.

I claim:

1. In the process for electro-depositing thick plates of bright rhodium onto metal using an acid electrolyte containing at least one rhodium compound from the group consisting of rhodium sulfate and rhodium phosphate in an amount sufiicient. to provide said electrolyte with about 0.5 to about 5 grams per liter of rhodium, free acid from the group consisting of sulfuric acid and phosphoric acid in an amount suflicient to 1 vent hydroysis of the rhodium compound but not exceeding about 20 milliliters per liter of electrolyte with the balance essentially water; the improvement which comprises incorporating in said electrolyte about 0.001 to about 0.02 gram of lead as an acid-soluble lead compound whereby" thick, bright rhodium electro-deposits are obtained on metal surfaces by employing the aforesaid lead-containing acid electrolyte.

2. In the process for electro-depositing thick platesof bright rhodium onto metal using an acid electrolyte containing at least one rhodium compound from the group consisting of rhodium sulfate and rhodium phosphate in an amount sufiicient to provide said electrolyte with about 2 grams perliter of rhodium, free acid from the group consisting of sulfuric acid and phosphoric acid in an amount sufrlcient to prevent hydrolysis of the rhodium compound but not exceeding about 20 milliliters per liter of electrolyte with the balance essentially water; the im rovement which comprises incorporating in said electrolyte about 0.001 to about 0.02 gram cf lead as an acid-soluble lead compound whereby thick, bright rhodium electro-deposits are obtained on metal surfaces by employing the aforesaid lead-contalning acid electrolyte.

3. A process for the production of thick, bright electroplates or rhodium onto metal which comprises establishing a rhodium-plating electrolyte consisting essentially of an aqueous acid solution of a rhodium salt from the. group consisting of rhodium sulfate and rhodium phosphate such that said solution contains about 0.5 to 5 grams of rhodium per liter, acid from the group consisting of sulfuric acid and phosphoric acid in an amount suflicient to prevent hydrolysis of said rhodium but not exceeding more than about20 milliliters, per liter, about 0.061 to about 0.02 gram per liter of lead added as a soluble lead compound; and electrolytically depositing rhodium from said electrolyte onto said metal at a current density from about 2 to about 4 a l-- peres per square decimeter and at a temperature between about room temperature and about 66 C. to produce thick, bright, rhodium electro-depcsits upon said metal immersed as cathode in said electrolyte.

4. An electrolyte for use in obtaining bright electro-deposits of rhodium on metal which comprises an acid bath containing at least one rhodium compound from the group consisting of rhodium sulfate and rhodium phosphatelin an amount to provide about 0.5 to about 5 grams per liter of rhodium, acid from the group consisting of sulfuric acid and phosphoric acid in an amount suflicient to prevent hydrolysis of the rhodium compound but not exceeding about 20 milliliters per liter of electrolyte, at least one soluble lead compound in an amount toprovide aboutl0.001 to about 0.02 gram per liter of lead, with the balance essentially water, whereby thick, brightv rhodium electro-deposits can be obtained on metal surfaces when employing the aforesaid lead-containing aqueous acid bath in electro-depositing rhodium onto metal surfaces.

5. An electrolyte for use in obtaining bright electro-deposits of rhodium on metal which consists essentially of an aqueous acid solutions of a material from the group consisting of rhodium sulfate and rhodium phosphate in an amount provide about 2 grams per liter of rhodium, acid from the group'consisting ofsulfuric acid and phosphoric acid in an amount sufilcient to preventhydrolysis of the rhodium compound but 5 6 not exceeding about 20- milliliters per liter of UNITED STATES PATENTS electrolyte, and at least one soluble lead com Number Name Date pound in an amount to provide about 0.001 to 1,981,820 Zimmerman 20, 1934 about 0.02 gram per liter of lead. 2,119,304 viers et aL May 31, 1933 FRANK E BERT REID. 5 2,250,556 Stareck July 29, 1941 2,401,331 Brenner et a1. June 4, 1946 fiiferences are of record in the Transactions of the Electrochemical Society,

6 13 pa en 10 vol. so (1941), pp. 494 to 497. 

1. IN THE PROCESS FOR ELECTRO-DEPOSITING THICK PLATES OF BRIGHT RHODIUM ONTO METAL USING AN ACID ELECTROLYTE CONTAINING AT LEAST ONE RHODIUM COMPOUND FROM THE GROUP CONSISTING OF RHODIUM SULFATE AND RHODIUM PHOSPHATE IN AN AMOUNT SUFFICIENT TO PROVIDE SAID ELECTROLYTE WITH ABOUT 0.5 TO ABOUT 5 GRAMS PER LITER OF RHODIUM, FREE ACID FROM THE GROUP CONSISTING OF SULFURIC ACID AND PHOSPHORIC ACID IN AN AMOUNT SUFFICIENT TO PREVENT HYDROYSIS OF THE RHODIUM COMPOUND BUT NOT EXCEEDING ABOUT 20 MILLILITERS PER LITER OF ELECTROLYTE WITH THE BALANCE ESSENTIALLY WATER; THE IMPROVEMENT WHICH COMPRISES INCORPORATING IN SAID ELECTROLYTE ABOUT 0.001 TO ABOUT 0.02 GRAM OF LEAD AS AN ACID-SOLUBLE LEAD COMPOUND WHEREBY THICK, BRIGHT RHODIUM ELECTRO-DEPOSITS ARE OBTAINED ON METAL SURFACES BY EMPLOYING THE AFORESAID LEAD-CONTAINING ACID ELECTROLYTE. 